1. Field of the Invention
The present invention relates to semiconductor integrated circuit (IC) manufacturing, and more specifically, to a mask with sub-wavelength diffractive elements to reduce corner rounding and a method of designing such a mask with sub-wavelength diffractive elements to reduce corner rounding.
2. Discussion of Related Art
Improvements in photolithography have allowed higher density and faster speed to be attained in integrated circuits (ICs) by continually shrinking the devices in a chip. According to a Rayleigh criterion, a smallest dimension that can be resolved by an imaging tool is directly proportional to a wavelength of exposure light and inversely proportional to the numerical aperture (NA) of the projection optics. However, diffraction may degrade the aerial image when a critical dimension (CD) becomes smaller than the wavelength of the light used to expose a photoresist film on a wafer. The exposure light may include deep ultraviolet (DUV) light with a wavelength of 248 nanometers (nm) or 193 nm.
Photolithography in the sub-wavelength regime will benefit from wavefront engineering using a resolution enhancement technique (RET), such as a phase-shifting mask (PSM), to achieve a sufficiently wide process latitude.
However, as the CD is scaled down, a mask may become unable to print the product features accurately enough. Consequently, other techniques may be needed to enhance fidelity of the pattern printed on the wafer.
In particular, optical proximity correction (OPC) may be used to modify the product features in the patterns on the mask to compensate for variations and non-uniformities in the fabrication process for the layer of the chip.
When traditional OPC is applied to the design of the mask, certain sub-resolution assist features, such as serifs and anti-serifs, may be used to modify the edges of the product features. Other sub-resolution assist features, such as scattering bars, may be placed close by to the product features.
Empirically-derived rules may be formulated for OPC to help define product features that cannot otherwise be reproduced reliably during manufacturing. However, as features shrink in size, many conflicts may arise in applying the rules. Consequently, a mask, such as PSM with OPC, may become complex to design.
Thus, what is needed is a mask with sub-wavelength diffractive elements to reduce corner rounding and a method of designing such a mask with sub-wavelength diffractive elements to reduce corner rounding.